A flat panel display device such as a liquid crystal display device is made up of a display area for carrying out image display, and a periphery area provided in the periphery of the display area. The display area designates an area for carrying out display of, for example, an image, and includes a display medium (for example, a liquid crystal layer in case of a liquid crystal display, or an EL layer in case of an EL display device) which generates an electro-optic effect or an electroluminescence effect. The display area further includes electric wiring provided on a substrate in a matrix manner for transmitting a display signal to the display medium. Meanwhile, the periphery area designates an outline area of the display area, and having periphery electric wiring required for the display, such as a leading electrode of the electric wiring provided in the display area in a matrix manner.
Generally, a driver IC for driving the display device is mounted to the periphery area by a COG (Chip on Glass) mounting method or a TCP (Tape Carrier Package) mounting method.
As one example, FIG. 8(a) shows a schematic view of a conventional liquid crystal display device which includes a driver IC mounted by a TCP mounting method. Further, FIG. 8(b) is a pattern diagram more minutely showing the arrangement of the TCP (a gate TCP 502 or a source TCP 503) of the liquid crystal display device shown in FIG. 8(a).
The display area on the lower substrate 500 of the liquid crystal display device includes signal wiring (gate wiring, source wiring: not shown) which is provided in a matrix manner. Further, an upper substrate 501 is provided on the signal wiring while covering the display area. The periphery area is an area of the lower substrate 500 not being covered by the upper substrate 501. An external circuit board 504 is provided outside of the periphery area for supplying an external signal. Further, a driver IC 505 is provided on the periphery area and the external circuit board 504 for operating as an intermediary, with plural TCPs (a gate TCP 502 or a source TCP503) for supplying a signal to the signal wiring on the display area. The external signal is supplied to the respective TCPs through the external circuit board 504.
As shown in FIG. 8(b), each of the TCPs 503 includes a driver IC 505 provided on a flexible base 506, a signal input wiring 507 for supplying an external signal to the driver IC, a signal output wiring 508 for supplying the signal outputted from the driver IC 505 to the display area, and the like. The signal input wiring 507 is electrically conducted to a terminal provided on a terminal connection section 509 of the external circuit board 504. The signal output wiring 508 is electrically conducted to a terminal connection section 509 of the periphery area on the lower substrate 500.
Incidentally, in the liquid crystal display device using the TCP mounting method, a signal is individually and directly inputted to the respective TCPs 503 from the external circuit board 504, and therefore a great number of wirings are required on the external circuit board. This causes some problems such as complication of manufacturing, an increase in costs, or a decrease of reliability. In view of these problems, Japanese Laid-Open Patent Application Tokukaihei 04-313731/1992 (published on Nov. 5, 1992), and Japanese Laid-Open Utility Model Application Jitsukaihei 03-114820/1991 (published on Nov. 26, 1991) and some other applications disclose a liquid crystal display device using a so-called “signal propagation method” as a replacement of the TCP mounting method. In this signal propagation method, a signal is first inputted to one TCP, and then sequentially propagated to the adjacent TCP one after another.
The following will explain a liquid crystal display device adopting the signal propagation method, with reference to FIGS. 9(a) and 9(b). FIG. 9(a) is a schematic view of a liquid crystal display device adopting the signal propagation method, and FIG. 9(b) is a pattern diagram more minutely showing the arrangement of the TCP (a gate TCP 512 or a source TCP513) of the liquid crystal display device shown in FIG. 9(a).
As shown in FIG. 9(a), the liquid crystal display device includes a lower substrate 510, an upper substrate 511, plural source TCPs 513 and plural gate TCPs 512 provided in the periphery of the lower substrate 510, an external circuit board 514, and the like. Further, a driver IC 515 is provided in each of the TCPs.
Further, as shown in FIG. 9(b), each of the TCPs 513 (512) includes a driver IC 515 provided on a flexible base 522, an IC driving wiring 516 as a signal input wiring for supplying an external signal to the driver IC 515, a counter electrode wiring 517, a signal output wiring 518 for supplying an image signal outputted from the driver IC 515 to the display area, relay wirings 519 and 520 for supplying/outputting a driving signal to the next adjacent TCP, and the like. Note that, the respective wirings are connected to other wirings etc. in the terminal connection section 521.
The following will explain signal exchange among the TCPs in the liquid crystal display device using the signal propagation method. Firstly, an external signal is supplied from the external circuit board 514 to the driver IC in the first TCP via the IC driving wiring 516 and the counter electrode wiring 517. Secondly, the driver IC transmits an image signal according to the external signal to the display area via the signal output wiring 518; and a part of the external signal is transmitted to the relay wirings 519 and 520. The signal transmitted to the relay wirings 519 and 520 is outputted to the relay wirings 519 and 520 in the second TCP via the connection wiring, which provides conduction among the TCPs, and further supplied to the driver IC of the second TCP. As described, in the foregoing method, a signal is first supplied to the TCP from the external circuit board 514, and a part of the signal is then supplied to a pixel of the display area through the driver IC in the TCP. The remaining signals are sequentially propagated to the next adjacent TCP via the relay wiring of the TCP and the connection wiring on the substrate.
With this signal propagation method, the number of wirings required for the input operation from the external circuit board to the TCP can be greatly reduced compared to a conventional TCP mounting method, thus realizing a simpler configuration, a smaller area, and cost reduction with respect to the external circuit board. Further, the publication Tokukaihei 04-313731/1992 and the publication Jitsukaihei 03-114820/1991 also disclose a structure not requiring the external circuit board by providing a bus line (connection wiring among the TCPs) in the periphery area of the display panel.
However, in the arrangements disclosed in the foregoing two publications, the bus line is excessively long and therefore the resistance of the connection wiring increases. Further, the wiring on the liquid crystal panel is generally formed on a glass substrate with a thin film having thickness of not more than 5 μm. Therefore, the resistance of the wiring becomes much higher than that of the external circuit board or that of the TCP, thus causing problems of such as delay of signal propagation. Particularly, the high resistance can cause a decrease of such as power source voltage for driving the liquid crystal driving IC of the TCP, and/or a decrease of power source voltage for driving the counter common electrode. If these power source voltages greatly decrease, there arises some operational problems.
In order to prevent the high resistance, the foregoing structure has to be provided with an external circuit board so as to individually input a signal to the respective TCPs, since the signal propagation is carried out under low resistance condition.
Meanwhile, Japanese Laid-Open Patent Application Tokukaihei 06-13724/1994 (published on Jan. 21, 1994) uses a bus line made of wiring (metal film) adhered to a glass substrate of a display device by transferring, instead of the wiring formed on a glass substrate by thin film forming technology. With this arrangement, the wiring can be greater than 0.5 μm (preferably in a range of to 10 μm), thus decreasing the resistance of the bus line. As a result, it is possible to provide a bus line which traverses over the whole periphery section (periphery area) of the display panel, thus realizing a display panel not using an external circuit board.
Incidentally, in the display device disclosed in the publication Tokukaihei 06-13724/1994, the bus line between the respective TCPs is made of a transferred metal film having a large thickness for the purpose of reducing the resistance; on the other hand, the leading wiring to the display area is made of the conventional thin film wiring formed by the thin film forming technology. Thus, the periphery area of the display device includes both wiring having a large thickness and wiring having a small thickness, i.e., the connection wiring as the bus line between the TCPs, and the leading wiring to the display area.
FIG. 10 is a cross-sectional view of a wiring substrate provided in a periphery area of a display device and having two wirings different in thickness. As shown in the figure, when the periphery area includes both connection wiring 531 made of a transferred metal film and leading wiring 532 made of a metal thin film, connection failure is likely to occur between connection terminals 533 formed on a TCP 534, and the respective wirings 531 and 532 formed on a glass substrate 535.
More specifically, when the TCP 534 and the glass substrate 535 are connected via an isotropic conductive adhesion material 536, conduction between the upper and lower wirings can be obtained in the portion of the connection wiring 531 having a large thickness since conductive particles included in the isotropic conductive adhesion material 536 can be sufficiently pressed. However, the leading wiring 532 having a small thickness cannot sufficiently press the conductive particles, thus causing connection failure in the portion of the leading wiring 532.